When telecom operators sell DSL to customers, it is a problem that the properties of the telecommunications line to the customer are not sufficiently well known. Because of that, it may not be possible to predict how much DSL capacity (e.g. number of Mbits/second) that the line can support, and hence that can be sold to the customer.
To be able to predict the DSL capacity that can be supported, it is useful to know the values of the line attenuation for the used frequencies. Usually, it is sufficient to know only the magnitude and not the phase. Line attenuation varies with the frequency, so it is usually necessary to know the attenuation for each, or most, of the used frequencies.
If the line attenuation for each frequency is known with sufficient accuracy, and also the line noise (PSD, power spectral density), it is possible to estimate from these the achievable DSL bit rate on the line.
Preferably, any line measurements of line properties should be made using single-ended line testing (SELT), which can be carried out from the operator's premises.
Double-ended line testing requires equipment to be present also at the customer end of the line. Sending technicians to the customer site is expensive, and before deciding to subscribe to a DSL service, customers usually do not have any DSL modem or other equipment that could assist in making a double-ended line test.
Hence, it is desirable to be able to estimate the magnitude of line attenuation by using SELT from the operator premises.
One previous way of estimation is to estimate the length of the line by measuring the arrival time of a time domain reflectometry (TDR) far-end echo of the line. Then, from a standard value for each frequency of attenuation per unit of length of line, attenuation for the particular line is estimated. This method yields unsatisfactory accuracy, likely because attenuation per length unit differs between cable types. It is often not known in advance what type or types of cable that the line is made from.
In the patent application US20050057880A1 is disclosed a method in which a pulse of narrow bandwidth is sent to a line to be measured. A far-end reflection is identified and the line attenuation for the used frequency band is determined directly from the amplitude ratio of the reflected and the sent pulse. An attenuation so determined is in general valid only for the used frequency band of the pulse. It is necessary to determine the magnitude of both the sent and the received pulse. The method is not suitable for implementation in a line card because of the influence on the signal of the line card transceiver.